There have been attempts in the mid 70's to trigger lightning by sending a powerful infrared laser beam through the atmosphere [Jr. C. W. Schubert. The Laser Lightning Rod System: a Feasibility Study. Technical Report AFFDL-TR-78-60, Air Force Flight Dynamics Laboratory, 1977]. Because of the long wavelength of these lasers, it would have taken a 100-photon process to ionize the air. However, the field of the intense laser beam was sufficient to create avalanche ionization of air, leading to a plasma. The method failed, because the dense plasma created is opaque to the beam, and prevented further propagation of the laser radiation.
Lightning between ground and cloud can be triggered by a inserting a conducting path to ground above the naturally-occurring space charge layer at a speed higher than the speed of space-charge ions. When this happens, space charge can no longer reduce the field at the tip of the conductor, so the enhanced fields at the tip can be large enough to initiate breakdown, with a resulting leader propagating from the ground to the cloud. A number of investigators have used this principle to trigger lightning by firing small rockets, pulling up grounded wires [M. M. Newman, J. R. Stahmann, J. D. Robb, E. A. Lewis, S. G. Martin, and S. V Zinn. Triggered lightning at close range. J. Geophys. Res., 72, 1967.
The success rate of this method is only about 50%. The speed of the rocket is not always sufficient to prevent formation of space charges that reduce the field between the cloud and the rocket. Finally, rockets cannot be fired continuously, as a laser can.